Sealing between rotors and stators in hostile environments such as exist, for example, in gas or steam turbines is well known to be extremely challenging. In recent years there have been proposals to replace the brush seals, which have become the industry standard, with so-called strip or leaf seals. In such seals an array of adjacent leaves are mounted in an annular carrier to project radially inwardly with their planes lying parallel to the axis of the carrier. In general these seals are designed so that when the rotor rotates the free tips of the leaves experience hydro-dynamic lift so that the leaves are held just off the surface of the rotor in a manner which provides good sealing, whilst reducing wear. However, it has been discovered that if the pressure distribution in the leaf seal, taken in an axial direction, is asymmetric, then the lifting force can become unbalanced from the optimal condition, with the leaf tips either not being lifted clear of the rotor or the gap between the rotor and the leaf tips becoming too large.
It has been proposed by H Nakane et al in their paper “The Development Of High Performance Leaf Seals” published in the Proceedings of ASME TURBO EXPO 2002 to achieve a symmetric pressured distribution within the leaf seal by ensuring that the gaps between the leaves and the respective front and back plates (or high pressure and low pressure side plates) are equal. This is theoretically a perfectly acceptable solution, but in practice it is very difficult to achieve and maintain accurate location, particularly on a production basis.